The Language Programs Use to Talk

An API (Application Programming Interface) is a set of rules that allows one program to communicate with another. In web development, we most commonly work with REST APIs.

Core REST Principles

1. Resources

Everything in a REST API is a resource — users, posts, products — each accessible via its own URL:

• GET /api/users — get all users
• GET /api/users/42 — get user with ID 42
• POST /api/users — create a new user

2. HTTP Methods

Method	Purpose
GET	Retrieve data (safe, no side effects)
POST	Create a new resource
PUT	Replace an existing resource entirely
PATCH	Partially update a resource
DELETE	Remove a resource

3. HTTP Status Codes

Servers communicate success/failure via status codes:

• 200 OK — success
• 201 Created — new resource created
• 404 Not Found — resource doesn't exist
• 401 Unauthorized — you need to log in
• 500 Internal Server Error — something broke on the server